Heat pipe lobar wicking arrangement

ABSTRACT

A heat pipe has the usual central wick for transporting a liquid, such as a cryogenic liquid, from end to end. A secondary wick transports liquid radially between the central wick and the wall of the heat pipe. This secondary wick is formed with lobes for contacting the heat pipe walls. In this manner, much of the wall surface is free of any contact with the wick, thereby providing improved heat transfer characteristics.

United States Patent [1 1 Nelson HEAT PIPE LOBAR WICKING ARRANGEMENT[75] Inventor: Burke Edward Nelson, Ridgefield,

Conn.

[73] Assignee: The Perkin-Elmer Corporation,

Norwalk, Conn.

{22] Filed: July 9, 1973 [21] Appl. No.: 377,728

[52] U5. Cl 165/105; 29/1573 R; 138/40 [51] Int. Cl. F28d 15/00 [58]Field of Search 165/105; 138/40; 29/1573 R [56] References Cited UNITEDSTATES PATENTS 3,283,787 11/1966 Davis 138/148 [111 3,892,273 [451 July1,1975

3,620,298 11/1971 Somerville et al. 165/[05 3,720,988 3/1973 Waters165/105 X 3,734,173 5/1973 Mon'tz 165/105 Primary Examiner-Albert W.Davis, Jr. Attorney, Agent. or Firm-John K. Conant 5 7 ABSTRACT A heatpipe has the usual central wick for transporting a liquid, such as acryogenic liquid, from end to end. A secondary wick transports liquidradially between the central wick and the wall of the heat pipe. Thissecondary wick is formed with lobes for contacting the heat pipe walls.In this manner, much of the wall surface is free of any contact with thewick thereby providing improved heat transfer characteristics.

5 Claims, 5 Drawing Figures HEAT PIPE LOBAR WICKING ARRANGEMENTBACKGROUND OF THE INVENTION This invention is a heat pipe for use withliquids, particularly cryogenic liquids, wherein the liquid evapo ratesat one end of the pipe and condenses at the other. The liquid is movedfrom the condenser region to the evaporator region by capillaryattraction along a central wick. A secondary wick in the condenser endtransports liquid from the inner walls of the tube to the central wickand a similar secondary wick in the evaporator end transports the liquidfrom the central wick to the walls. In heat pipes constructed inaccordance with the prior art, it is customary for the secondary wick tolie against the inner wall of the heat pipe. One example of such aconstruction will be found in US. Pat. No. 3,720,988 of Waters. Theproblem with such a construction arises from the fact that cryogenicliquids have very poor thermal conductivity. The heat of vaporizationmust traverse not only the wall of the heat pipe but also a layer ofwick and a layer of the cryogenic liquid. This presents a path of highthermal resistance between the outer surface of the heat pipe and thepoint of evaporation on the inside of the screen wick. Accordingly, itis the primary object of the present invention to provide a constructionwhich permits efficient wicking but reduces the thermal resistance ofthe heat path. Other objects, features, and advantages will be apparentfrom the following description and appended claims.

SUMMARY OF THE INVENTION A heat pipe is formed with a thermallyconductive tube having threads or grooves on its inner surface and aliquid, particularly a cryogenic liquid, contained with the tube. Aprimary wick extends substantially axially through the tube to transportliquid therealong. A secondary wick is provided which has a plurality ofsubstantially radial portions extending between the primary wick and theinner surface of the heat pipe. These portions contact the inner surfaceat circumferentially spaced discrete regions so as to transport iiquidbetween the primary wick and the inner surface.

BRIEF DESCRIPTION OF THE DRAWINGS With particular reference to thedrawings:

FIG. I is a perspective view of the end of a heat pipe having a wickconstructed in accordance with the present invention, portions thereofbeing broken away to better illustrate the construction;

FIG. 2 is an enlarged cross section taken through the outer wall of theheat pipe of FIG. 1;

FIG. 3 is a perspective view showing one manner in which the wick of theinvention may be constructed;

FIG. 4 is a perspective view showing the wick of FIG. 3 positionedwithin a heat pipe; and

FIG. 5 illustrates another embodiment ofa wick constructed in accordancewith the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS With particular reference toFIGS. 1 and 2 there is illustrated a heat pipe constructed in accordancewith the present invention with the end cap removed. It comprises ametallic tube 10, having circumferential grooves or threads 12 formed onits internal surface. Extending axially the length ofthe heat pipe is acentral wick [4 formed of a fine mesh stainless steel screen which hasbeen spirally wrapped around a central core, such as a steel rod. In theillustrated heat pipe the central core is removed. However, it may beleft in place if desired. Secondary wicking between the central wick andthe side wall is provided by a similar fine mesh screen, spot welded tothe central wick 14 so as to form lobes l6 a-d. The lobes are slightlylonger than the distance from central wick 14 to the side wall and are,therefore, slightly deformed when inserted into tube 10. The four lobesengage the inside wall at discrete circumferentially spaced regions,thereby leaving large portions of the side wall out of contact with thewick. The natural resilience of the screen keeps the central wick l4centrally located within the heat pipe. While the lobes l6 a-d mayextend the entire length of the heat pipe, it is not necessary that theydo so as their primary function is the transport of liquid between thecentral wick l4 and the tube 10 in the evaporator and condenser regions.Between these regions the lobes may be removed, if desired.

It will be understood that the materials and dimensions may vary withthe proposed application and the liquid employed. However, in onesuccessful embodiment utilizing liquid nitrogen as the liquid, the tube10 had an external diameter of I inch and a wall thickness of 0.235centimeters. There were 24 threads per inch with a depth of 0.0675centimeters and a flat dimension at the root of 0.013 centimeters. Thecentral wick 14 had a diameter of 0.3 inch and both it and the lobeswere formed of stainless steel screen of 1,500 mesh.

Assuming FIG. 1 to illustrate the evaporator region of a heat pipe, theliquid nitrogen is transported from the remote condenser region bycapillary attraction in the central wick 14. It is then transferredradially to the wall by the wicking action of the lobes 16. The threads12 also provide wicking so that by capillary attraction the liquid 18 ispulled into the thread channels, as shown in FIG. 2, where it isevaporated. The fine channels formed by the threads prevent a continuouslayer of the poor thermally conductive cryogenic liquid from forming onthe walls. The flow is, of course, reversed in the condensing region ofthe heat pipe.

The four lobe arrangement illustrated in FIG. 1 is primarily for use inground testing in a gravitational environment. As liquid will naturallytend to migrate to the bottom of the heat pipe, it is desirable to havea lobe there and it is also desired to have lobes on the side wall topick up the fluid as it flows down under the influence of gravity. Inthe evaporator the opposite is true. In order for the upper quadrants ofthe wall to be wetted, liquid must be delivered to the upper edge sothat it can flow down under the influence of gravity. For these reasons,lobes are desired at the top, bottom, and each side.

In a gravity-free space environment, three lobes may be more desirableand such a construction is illustrated in FIGS. 3 and 4. Furthermore, ifdesired, the central wick may have a mesh size differing from that ofthe lobes. FIG. 3 illustrates the construction of such a wick, wherein ascreen 20 is spirally wrapped to form central wick 22. Another screen 24of a different mesh size is interleaved with the outer layer of thecentral wick and is spot welded thereto leaving lobes 260-0. The lobesare then bent over and the entire wick inserted into a tube 28 with arotating motion as shown in FIG. 4.

In the FIGS. 1 and 4 embodiments, the lobes are deformed into a swastikaeffect, However, they could also take the form of ellipses compressedalong their major axes.

A different construction is illustrated in FIG. 5. In this embodiment asimilar internally threaded tube 30 is employed. However, the wick32 isa solid formed of sintered metal having a pore: size suchf as to providethe required capillary action. The wick 32 is of cruciform cross-sectionand the ends of the cross arms have external threads 34 which match thethreads in tube 30. The resulting criciform wick is threaded into thetube 30 and the arms function as lobes to radially transport thecryogenic liquid.

It is believed that the many advantages of this invention will now beapparent to those skilled in the art. It will also be apparent that anumber of variations and modifications may be made therein withoutdeparting from its spirit and scope. Accordingly, the foregoingdescription is to be construed as illustrative only, rather thanlimiting. This invention is limited only by the scope of the followingclaims.

I claim:

l. A heat pipe comprising: a thermally conductive tube havingcircumferential grooves in its inner surface; a liquid contained withinsaid tube; a primary wick extending substantially axially through saidtube to transport said liquid therealong; and a secondary wickcomprising a screen formed into radial lobes deformed between saidprimary wick and said inner surface, said lobes contacting said innersurface at circumferentially spaced, discrete regions thereof totransport said liquid between said primary wick and said inner surface.

2. The heat pipe of claim 1 wherein said central wick comprises amulti-layered screen.

3. The wick of claim 2 wherein the mesh size of the secondary wick isdifferent from that of the central wick.

4. The heat pipe of claim 1 in which said secondary wick comprises fourof said lobes, two extending radially from opposite sides of the primarywick, respectively, in a first plane and the other two extending fromopposite sides of the primary wick in a second plane that issubstantially normal to said first plane.

5. The heat pipe of claim I in which said primary wick comprises threeof said lobes extending radially from the primary wick at points spacedsubstantially equal distances apart around the circumference of theprimary wick.

1. A heat pipe comprising: a thermally conductive tube havingcircumferential grooves in its inner surface; a liquid contained withinsaid tube; a primary wick extending substantially axially through saidtube to transport said liquid therealong; and a secondary wickcomprising a screen formed into radial lobes deformed between saidprimary wick and said inner surface, said lobes contacting said innersurface at circumferentially spaced, discrete regions thereof totransport said liquid between said primary wick and said inner surface.2. The heat pipe of claim 1 wherein said central wick comprises amulti-layered screen.
 3. The wick of claim 2 wherein the mesh size ofthe secondary wick is different from that of the central wick.
 4. Theheat pipe of claim 1 in which said secondary wick comprises four of saidlobes, two extending radially from opposite sides of the primary wick,respectively, in a first plane and the other two extending from oppositesides of the primary wick in a second plane that is substantially normalto said first plane.
 5. The heat pipe of claim 1 in which said primarywick comprises three of said lobes extending radially from the primarywick at points spaced substantially equal distances apart around thecircumference of the primary wick.